同步和异步请求
同步：一步步来，单线程
异步：在没有加载完就执行后面的代码，所以要注册请求状态用以监视来执行回调函数（服务器接收了请求如何响应）
var request = new XMLHttpRequest(); //创建对象
request.open('GET', 'https://developer.mozilla.org/', false); //初始化同步请求
request.send(null);//发送请求
if (request.status === 200) {
console.log(request.responseText);
}//回调函数
request.open('GET', 'https://developer.mozilla.org/', false); //初始化同步请求
request.send(null);//发送请求
if (request.status === 200) {
console.log(request.responseText);
}//回调函数
>undefinedGET
https://developer.mozilla.org/ [HTTP/1.1 301 MOVED PERMANENTLY 1449ms]
GET
https://developer.mozilla.org/zh-CN/ [HTTP/1.1 200 OK 1176ms]
"..."
var request = new XMLHttpRequest();
request.open('GET', 'https://developer.mozilla.org/', true); //异步请求
request.send(null);
if (request.status === 200) {
console.log(request.status);
}
request.open('GET', 'https://developer.mozilla.org/', true); //异步请求
request.send(null);
if (request.status === 200) {
console.log(request.status);
}
>undefinedGET
var request = new XMLHttpRequest();
request.onreadystatechange = mantou;//注册状态监视函数
request.open('GET', 'https://developer.mozilla.org/', true); //异步请求
request.send(null);
function mantou(){if (request.status === 200) {
console.log(request.status);
}}
request.onreadystatechange = mantou;//注册状态监视函数
request.open('GET', 'https://developer.mozilla.org/', true); //异步请求
request.send(null);
function mantou(){if (request.status === 200) {
console.log(request.status);
}}
>undefinedGEThttps://developer.mozilla.org/zh-CN/ [HTTP/1.1 200 OK 8424ms]200